Molecular cloning of the Golgi apparatus uridine diphosphate-N-acetylglucosamine transporter from Kluyveromyces lactis

Delivery of Nucleotide Sugars to the Mammalian Golgi: A Very Well (un)Explained Story

Nucleotide sugars (NSs) serve as substrates for glycosylation reactions. The majority of these compounds are synthesized in the cytoplasm, whereas glycosylation occurs in the endoplasmic reticulum (ER) and Golgi lumens, where catalytic domains of glycosyltransferases (GTs) are located. Therefore, translocation of NS across the organelle membranes is a prerequisite. This process is thought to be mediated by a group of multi-transmembrane proteins from the SLC35 family, i.e., nucleotide sugar transporters (NSTs). Despite many years of research, some uncertainties/inconsistencies related with the mechanisms of NS transport and the substrate specificities of NSTs remain. Here we present a comprehensive review of the NS import into the mammalian Golgi, which consists of three major parts. In the first part, we provide a historical view of the experimental approaches used to study NS transport and evaluate the most important achievements. The second part summarizes various aspects of knowledge concerning NSTs, ranging from subcellular localization up to the pathologies related with their defective function. In the third part, we present the outcomes of our research performed using mammalian cell-based models and discuss its relevance in relation to the general context.

Characterisation of a gene cluster involved in aspergillus fumigatus zwitterionic glycosphingolipid synthesis

The human pathogenic fungus Aspergillus fumigatus synthesises the zwitterionic glycolipid Manα1,3Manα1,6GlcNα1,2IPC, named Af3c. Similar glycosphingolipids having a glucosamine (GlcN) linked in α1,2 to inositolphosphoceramide (IPC) as core structure have only been described in a few pathogenic fungi. Here, we describe an Ammophilus fumigatus cluster of 5 genes (AFUA_8G02040 to AFUA_8G02090) encoding proteins required for the glycan part of the glycosphingolipid Af3c. Besides the already characterised UDP-GlcNAc:IPC α1,2-N-acetylglucosaminyltransferase (GntA), the cluster encodes a putative UDP-GlcNAc transporter (NstA), a GlcNAc de-N-acetylase (GdaA), and two mannosyltransferases (OchC and ClpC). The function of these proteins was inferred from analysis of the glycolipids extracted from A. fumigatus strains deficient in one of the genes. Moreover, successive introduction of the genes encoding GntA, GdaA, OchC and ClpC in the yeast Saccharomyces cerevisiae enabled the reconstitution of the Af3c biosynthetic pathway. Absence of Af3c slightly reduced the virulence of A. fumigatus in a Galleria mellonella infection model.

Biosynthesis of GlcNAc-rich N- and O-glycans in the Golgi apparatus does not require the nucleotide sugar transporter SLC35A3

Nucleotide sugar transporters, encoded by the SLC35 gene family, deliver nucleotide sugars throughout the cell for various glycosyltransferase-catalyzed glycosylation reactions. GlcNAc, in the form of UDP-GlcNAc, and galactose, as UDP-Gal, are delivered into the Golgi apparatus by SLC35A3 and SLC35A2 transporters, respectively. However, although the UDP-Gal transporting activity of SLC35A2 has been clearly demonstrated, UDP-GlcNAc delivery by SLC35A3 is not fully understood. Therefore, we analyzed a panel of CHO, HEK293T, and HepG2 cell lines including WT cells, SLC35A2 knockouts, SLC35A3 knockouts, and double-knockout cells. Cells lacking SLC35A2 displayed significant changes in N- and O-glycan synthesis. However, in SLC35A3-knockout CHO cells, only limited changes were observed; GlcNAc was still incorporated into N-glycans, but complex type N-glycan branching was impaired, although UDP-GlcNAc transport into Golgi vesicles was not decreased. In SLC35A3-knockout HEK293T cells, UDP-GlcNAc transport was significantly decreased but not completely abolished. However, N-glycan branching was not impaired in these cells. In CHO and HEK293T cells, the effect of SLC35A3 deficiency on N-glycan branching was potentiated in the absence of SLC35A2. Moreover, in SLC35A3-knockout HEK293T and HepG2 cells, GlcNAc was still incorporated into O-glycans. However, in the case of HepG2 cells, no qualitative changes in N-glycans between WT and SLC35A3 knockout cells nor between SLC35A2 knockout and double-knockout cells were observed. These findings suggest that SLC35A3 may not be the primary UDP-GlcNAc transporter and/or different mechanisms of UDP-GlcNAc transport into the Golgi apparatus may exist.

Nucleotide sugar transporters of Entamoeba histolytica and Entamoeba invadens involved in chitin synthesis

Chitin, a homopolymer of β-(1,4) linked N-acetylglucosamine (GlcNAc), is a major component of cyst wall in the protozoan parasites Entamoeba histolytica (Eh) and Entamoeba invadens (Ei). The Entamoeba chitin synthase makes chitin at the vesicular membrane rather than the plasma membrane in fungi, even though the chemistry of chitin synthesis is most likely the same. However, the role of nucleotide sugar transporter(s) (NSTs) that are involved in chitin synthesis in Entamoeba are not yet established. In this study, we have identified the putative UDP-GlcNAc transporter (EiNst5) of Ei by BLASTP analysis using the amino acid sequence of EhNst3, the UDP-GlcNAc transporter of Eh. Heterologous expression of both EhNst3 and EiNst5 was found to complement the function of Yea4p (UDP-GlcNAc transporter of S. cerevisiae) in YEA4 null mutant and increased the cell wall chitin content. Like Yea4p in S. cerevisiae, Myc-epitope tagged EhNst3 and EiNst5 were localized to the endoplasmic reticulum in Δyea4 cells. The EiNST5 transcript was up-regulated during the in vitro encystation and oxidative stress in E. invadens. Similar up-regulation was also seen for EhNST3 under oxidative stress in E. histolytica. Down-regulation of EiNst5 expression using gene-specific dsRNA significantly reduced cyst formation during in vitro encystation in E. invadens. Our observations suggest for the first time the involvement of EhNst3 and EiNst5 in chitin synthesis and so in encystation of Entamoeba.

Tailoring N-Glycan Biosynthesis for Production of Therapeutic Proteins in Saccharomyces cerevisiae

The ability to control and adjust the N-glycosylation pathway of Saccharomyces cerevisiae is a key step toward production of therapeutic glycoproteins such as antibodies or erythropoietin. The focus of this chapter is to describe the road from yeast-type N-glycosylation to human-type complex N-glycosylation. The chapter describes the cell engineering and provides the detailed analytical procedures required to perform glycan analysis using MALDI-TOF mass spectrometry.

Conserved Glu-47 and Lys-50 residues are critical for UDP-N-acetylglucosamine/UMP antiport activity of the mouse Golgi-associated transporter Slc35a3

Nucleotide sugar transporters (NSTs) regulate the flux of activated sugars from the cytosol into the lumen of the Golgi apparatus where glycosyltransferases use them for the modification of proteins, lipids, and proteoglycans. It has been well-established that NSTs are antiporters that exchange nucleotide sugars with the respective nucleoside monophosphate. Nevertheless, information about the molecular basis of ligand recognition and transport is scarce. Here, using topology predictors, cysteine-scanning mutagenesis, expression of GFP-tagged protein variants, and phenotypic complementation of the yeast strain Kl3, we identified residues involved in the activity of a mouse UDP-GlcNAc transporter, murine solute carrier family 35 member A3 (mSlc35a3). We specifically focused on the putative transmembrane helix 2 (TMH2) and observed that cells expressing E47C or K50C mSlc35a3 variants had lower levels of GlcNAc-containing glycoconjugates than WT cells, indicating impaired UDP-GlcNAc transport activity of these two variants. A conservative substitution analysis revealed that single or double substitutions of Glu-47 and Lys-50 do not restore GlcNAc glycoconjugates. Analysis of mSlc35a3 and its genetic variants reconstituted into proteoliposomes disclosed the following: (i) all variants act as UDP-GlcNAc/UMP antiporters; (ii) conservative substitutions (E47D, E47Q, K50R, or K50H) impair UDP-GlcNAc uptake; and (iii) substitutions of Glu-47 and Lys-50 dramatically alter kinetic parameters, consistent with a critical role of these two residues in mSlc35a3 function. A bioinformatics analysis revealed that an EXXK motif in TMH2 is highly conserved across SLC35 A subfamily members, and a 3D-homology model predicted that Glu-47 and Lys-50 are facing the central cavity of the protein.

My journey in the discovery of nucleotide sugar transporters of the Golgi apparatus

Defects in protein glycosylation can have a dramatic impact on eukaryotic cells and is associated with mental and developmental pathologies in humans. The studies outlined below illustrate how a basic biochemical problem in the mechanisms of protein glycosylation, specifically substrate transporters of nucleotide sugars, including ATP and 3'-phosphoadenyl-5'-phosphosulfate (PAPS), in the membrane of the Golgi apparatus and endoplasmic reticulum, expanded into diverse biological systems from mammals, including humans, to yeast, roundworms, and protozoa. Using these diverse model systems allowed my colleagues and me to answer fundamental biological questions that enabled us to formulate far-reaching hypotheses and expanded our knowledge of human diseases caused by malfunctions in the metabolic processes involved.

SLC35A5 Protein-A Golgi Complex Member with Putative Nucleotide Sugar Transport Activity

Solute carrier family 35 member A5 (SLC35A5) is a member of the SLC35A protein subfamily comprising nucleotide sugar transporters. However, the function of SLC35A5 is yet to be experimentally determined. In this study, we inactivated the SLC35A5 gene in the HepG2 cell line to study a potential role of this protein in glycosylation. Introduced modification affected neither N- nor O-glycans. There was also no influence of the gene knock-out on glycolipid synthesis. However, inactivation of the SLC35A5 gene caused a slight increase in the level of chondroitin sulfate proteoglycans. Moreover, inactivation of the SLC35A5 gene resulted in the decrease of the uridine diphosphate (UDP)-glucuronic acid, UDP-N-acetylglucosamine, and UDP-N-acetylgalactosamine Golgi uptake, with no influence on the UDP-galactose transport activity. Further studies demonstrated that SLC35A5 localized exclusively to the Golgi apparatus. Careful insight into the protein sequence revealed that the C-terminus of this protein is extremely acidic and contains distinctive motifs, namely DXEE, DXD, and DXXD. Our studies show that the C-terminus is directed toward the cytosol. We also demonstrated that SLC35A5 formed homomers, as well as heteromers with other members of the SLC35A protein subfamily. In conclusion, the SLC35A5 protein might be a Golgi-resident multiprotein complex member engaged in nucleotide sugar transport.

An insight into the orphan nucleotide sugar transporter SLC35A4

SLC35A4 has been classified in the SLC35A subfamily based on amino acid sequence homology. Most of the proteins belonging to the SLC35 family act as transporters of nucleotide sugars. In this study, the subcellular localization of endogenous SLC35A4 was determined via immunofluorescence staining, and it was demonstrated that SLC35A4 localizes mainly to the Golgi apparatus. In silico topology prediction suggests that SLC35A4 has an uneven number of transmembrane domains and its N-terminus is directed towards the Golgi lumen. However, an experimental assay refuted this prediction: SLC35A4 has an even number of transmembrane regions with both termini facing the cytosol. In vivo interaction analysis using the FLIM-FRET approach revealed that SLC35A4 neither forms homomers nor associates with other members of the SLC35A subfamily except SLC35A5. Additional assays demonstrated that endogenous SLC35A4 is 10 to 40nm proximal to SLC35A2 and SLC35A3. To determine SLC35A4 function SLC35A4 knock-out cells were generated with the CRISPR-Cas9 approach. Although no significant changes in glycosylation were observed, the introduced mutation influenced the subcellular distribution of the SLC35A2/SLC35A3 complexes. Additional FLIM-FRET experiments revealed that overexpression of SLC35A4-BFP together with SLC35A3 and the SLC35A2-Golgi splice variant negatively affects the interaction between the two latter proteins. The results presented here strongly indicate a modulatory role for SLC35A4 in intracellular trafficking of SLC35A2/SLC35A3 complexes.

Functional expression of a human GDP-L-fucose transporter in Escherichia coli

OBJECTIVES

To investigate the translocation of nucleotide-activated sugars from the cytosol across a membrane into the endoplasmatic reticulum or the Golgi apparatus which is an important step in the synthesis of glycoproteins and glycolipids in eukaryotes.

RESULTS

The heterologous expression of the recombinant and codon-adapted human GDP-L-fucose antiporter gene SLC35C1 (encoding an N-terminal OmpA-signal sequence) led to a functional transporter protein located in the cytoplasmic membrane of Escherichia coli. The in vitro transport was investigated using inverted membrane vesicles. SLC35C1 is an antiporter specific for GDP-L-fucose and depending on the concomitant reverse transport of GMP. The recombinant transporter FucT1 exhibited an activity for the transport of ³H-GDP-L-fucose with a V_max of 8 pmol/min mg with a K_m of 4 µM. The functional expression of SLC35C1 in GDP-L-fucose overproducing E. coli led to the export of GDP-L-fucose to the culture supernatant.

CONCLUSIONS

The export of GDP-L-fucose by E. coli provides the opportunity for the engineering of a periplasmatic fucosylation reaction in recombinant bacterial cells.

Manganese ion concentration affects production of human core 3 O-glycan in Saccharomyces cerevisiae

BACKGROUND

Production of various mucin-like glycoproteins could be useful for development of antibodies specific to disease-related glycoproteins as well as for the biosynthesis of clinically useful glycoproteins. A Saccharomyces cerevisiae strain capable of in vivo production of mucin-type core 1 structure (Galβ1-3GalNAcα1-O-Ser/Thr) has been reported, but a strain producing core 3 structure (GlcNAcβ1-3GalNAcα1-O-Ser/Thr) has not been constructed.

METHODS

To generate core 3-producing strain, genes encoding uridine diphosphate (UDP)-Gal-4-epimerase, UDP-GalNAc transporter, UDP-GlcNAc transporter, and two glycosyltransferases were integrated into the genome. A Mucin-1-derived acceptor peptide (MUC1ap) was expressed as an acceptor. The amount of the resulting modified peptide was analyzed by HPLC.

RESULTS

Introduction of a codon-optimized UDP-GlcNAc:βGal β-1,3-N-acetylglucosaminyltransferase 6 (β3Gn-T6) gene yielded increases in β3Gn-T6 activity but did not alter the level of core 3 production. The highest in vitro activity of β3Gn-T6 was observed at Mn(2+) concentrations of 10mM and above. Supplementation of MnCl2 to the culture medium yielded increases of up to 25% in the accumulation of core 3 on the MUC1ap. The yeast invertase from the core 3-producing strain was less extensively N-glycosylated; however, it was partially restored by the addition of MnCl2 to the medium.

CONCLUSIONS

Physiological Mn(2+) concentration in S. cerevisiae was insufficient to facilitate optimal synthesis of core 3. Mn(2+) supplementation led to up-regulation of reaction of glycosylation in the Golgi, resulting in increases of core 3 production.

GENERAL SIGNIFICANCE

This study reveals that control of Mn(2+) concentration is important for production of specific mammalian-type glycans in S. cerevisiae.

Overview of Nucleotide Sugar Transporter Gene Family Functions Across Multiple Species

Glycoproteins and glycolipids are crucial in a number of cellular processes, such as growth, development, and responses to external cues, among others. Polysaccharides, another class of sugar-containing molecules, also play important structural and signaling roles in the extracellular matrix. The additions of glycans to proteins and lipids, as well as polysaccharide synthesis, are processes that primarily occur in the Golgi apparatus, and the substrates used in this biosynthetic process are nucleotide sugars. These proteins, lipids, and polysaccharides are also modified by the addition of sulfate groups in the Golgi apparatus in a series of reactions where nucleotide sulfate is needed. The required nucleotide sugar substrates are mainly synthesized in the cytosol and transported into the Golgi apparatus by nucleotide sugar transporters (NSTs), which can additionally transport nucleotide sulfate. Due to the critical role of NSTs in eukaryotic organisms, any malfunction of these could change glycan and polysaccharide structures, thus affecting function and altering organism physiology. For example, mutations or deletion on NST genes lead to pathological conditions in humans or alter cell walls in plants. In recent years, many NSTs have been identified and functionally characterized, but several remain unanalyzed. This study examined existing information on functionally characterized NSTs and conducted a phylogenetic analysis of 257 NSTs predicted from nine animal and plant model species, as well as from protists and fungi. From this analysis, relationships between substrate specificity and the primary NST structure can be inferred, thereby advancing understandings of nucleotide sugar gene family functions across multiple species.

A dual approach for improving homogeneity of a human-type N-glycan structure in Saccharomyces cerevisiae

N-glycosylation is an important feature of therapeutic and other industrially relevant proteins, and engineering of the N-glycosylation pathway provides opportunities for developing alternative, non-mammalian glycoprotein expression systems. Among yeasts, Saccharomyces cerevisiae is the most established host organism used in therapeutic protein production and therefore an interesting host for glycoengineering. In this work, we present further improvements in the humanization of the N-glycans in a recently developed S. cerevisiae strain. In this strain, a tailored trimannosyl lipid-linked oligosaccharide is formed and transferred to the protein, followed by complex-type glycan formation by Golgi apparatus-targeted human N-acetylglucosamine transferases. We improved the glycan pattern of the glycoengineered strain both in terms of glycoform homogeneity and the efficiency of complex-type glycosylation. Most of the interfering structures present in the glycoengineered strain were eliminated by deletion of the MNN1 gene. The relative abundance of the complex-type target glycan was increased by the expression of a UDP-N-acetylglucosamine transporter from Kluyveromyces lactis, indicating that the import of UDP-N-acetylglucosamine into the Golgi apparatus is a limiting factor for efficient complex-type N-glycosylation in S. cerevisiae. By a combination of the MNN1 deletion and the expression of a UDP-N-acetylglucosamine transporter, a strain forming complex-type glycans with a significantly improved homogeneity was obtained. Our results represent a further step towards obtaining humanized glycoproteins with a high homogeneity in S. cerevisiae.

Structure and function of nucleotide sugar transporters: Current progress

The proteomes of eukaryotes, bacteria and archaea are highly diverse due, in part, to the complex post-translational modification of protein glycosylation. The diversity of glycosylation in eukaryotes is reliant on nucleotide sugar transporters to translocate specific nucleotide sugars that are synthesised in the cytosol and nucleus, into the endoplasmic reticulum and Golgi apparatus where glycosylation reactions occur. Thirty years of research utilising multidisciplinary approaches has contributed to our current understanding of NST function and structure. In this review, the structure and function, with reference to various disease states, of several NSTs including the UDP-galactose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, GDP-fucose, UDP-N-acetylglucosamine/UDP-glucose/GDP-mannose and CMP-sialic acid transporters will be described. Little is known regarding the exact structure of NSTs due to difficulties associated with crystallising membrane proteins. To date, no three-dimensional structure of any NST has been elucidated. What is known is based on computer predictions, mutagenesis experiments, epitope-tagging studies, in-vitro assays and phylogenetic analysis. In this regard the best-characterised NST to date is the CMP-sialic acid transporter (CST). Therefore in this review we will provide the current state-of-play with respect to the structure–function relationship of the (CST). In particular we have summarised work performed by a number groups detailing the affect of various mutations on CST transport activity, efficiency, and substrate specificity.

Roles of the nucleotide sugar transporters (SLC35 family) in health and disease

Nucleotide sugars and adenosine 3'-phospho 5'-phosphosulfate (PAPS) are transported from the cytosol to the endoplasmic reticulum (ER) and the Golgi apparatus where they serve as substrates for the glycosylation and sulfation of proteins, lipids and proteoglycans. The translocation is accomplished by the nucleotide sugar transporters (NSTs), a family of highly conserved hydrophobic proteins with multiple transmembrane domains that are part of the solute carrier family 35 (SLC35). NSTs are antiporters responsible not only for transporting nucleotide sugars and PAPS into the Golgi, but also for the transport of the reaction products back to the cytosol. The initial reaction products - the nucleoside diphosphates - must be first converted to nucleoside monophosphates by a group of enzymes called ectonucleoside triphosphate diphosphohydrolases (ENTPDs) before they can exit the Golgi. The transport role of NSTs is essential to glycosylation and development. Mutations in two NST genes, SLC35A1 and SLC35C1, have been related to congenital disorder of glycosylation II (CDG II).

Isolation of Sporothrix schenckii GDA1 and functional characterization of the encoded guanosine diphosphatase activity

Sporothrix schenckii is a fungal pathogen of humans and the etiological agent of sporotrichosis. In fungi, proper protein glycosylation is usually required for normal composition of cell wall and virulence. Upon addition of precursor oligosaccharides to nascent proteins in the endoplasmic reticulum, glycans are further modified by Golgi-glycosyl transferases. In order to add sugar residues to precursor glycans, nucleotide diphosphate sugars are imported from the cytosol to the Golgi lumen, the sugar is transferred to glycans, and the resulting nucleoside diphosphate is dephosphorylated by the nucleoside diphosphatase Gda1 before returning to cytosol. Here, we isolated the open reading frame SsGDA1 from a S. schenckii genomic DNA library. In order to confirm the function of SsGda1, we performed complementation assays in a Saccharomyces cerevisiae gda1∆ null mutant. Our results indicated that SsGDA1 restored the nucleotide diphosphatase activity to wild-type levels and therefore is a functional ortholog of S. cerevisiae GDA1.

Golgi nucleotide sugar transporter modulates cell wall biosynthesis and plant growth in rice

Golgi-localized nucleotide sugar transporters (NSTs) are considered essential for the biosynthesis of wall polysaccharides and glycoproteins based on their characteristic transport of a large number of nucleotide sugars to the Golgi lumen. The lack of NST mutants in plants has prevented evaluation of this hypothesis in plants. A previously undescribed Golgi NST mutant, brittle culm14 (bc14), displays reduced mechanical strength caused by decreased cellulose content and altered wall structure, and exhibits abnormalities in plant development. Map-based cloning revealed that all of the observed mutant phenotypes result from a missense mutation in a putative NST gene, Oryza sativa Nucleotide Sugar Transporter1 (OsNST1). OsNST1 was identified as a Golgi-localized transporter by analysis of a fluorescence-tagged OsNST1 expressed in rice protoplast cells and demonstration of UDP-glucose transport activity via uptake assays in yeast. Compositional sugar analyses in total and fractionated wall residues of wild-type and bc14 culms showed a deficiency in the synthesis of glucoconjugated polysaccharides in bc14, indicating that OsNST1 supplies the glucosyl substrate for the formation of matrix polysaccharides, and thereby modulates cellulose biosynthesis. OsNST1 is ubiquitously expressed, with high expression in mechanical tissues. The inferior mechanical strength and abnormal development of bc14 plants suggest that OsNST1 has pleiotropic effects on cell wall biosynthesis and plant growth. Identification of OsNST1 has improved our understanding of how cell wall polysaccharide synthesis is regulated by Golgi NSTs in plants.

The effect of carbon source on the secretome of Kluyveromyces lactis

A proteomic analysis was performed on spent fermentation medium following bioreactor propagation of a wild-type industrial strain to identify proteins naturally secreted by Kluyveromyces lactis cells. Here, we report changes detected in the K. lactis secretome as a result of growth in three different carbon sources: glucose, galactose and glycerol. A total of 151 secreted proteins were detected by multi-dimensional separations and reversed-phase online nanoESI-MS/MS analysis. From these, we were able to identify 63 proteins (termed the "base secretome") that were common to all three fermentation conditions. The majority of base secretome proteins, 79%, possessed general secretory pathway (GSP) sequences and were involved with cell wall structure, glycosylation, carbohydrate metabolism and proteolysis. There was little variation in the functional groupings of base secretome GSP proteins and GSP proteins that were not part of the base secretome. In contrast, the majority of non-GSP proteins detected were not part of the base secretome and the functions of these proteins varied significantly. Finally, through further identification of non-GSP proteins in carbon sources not originally tested, we have gained further evidence of a protein export mechanism separate from the GSP in K. lactis.

Nucleotide sugar transporters of the Golgi apparatus

The Golgi apparatus is the major site of protein, lipid and proteoglycan glycosylation. The glycosylation enzymes, as well as kinases and sulfatases that catalyze phosphorylation and sulfation, are localized within the Golgi cisternae in characteristic distributions that frequently reflect their order in a particular pathway (Kornfeld and Kornfeld 1985; Colley 1997). The glycosyl-transferases, sulfotransferases and kinases are “transferases” that require activated donor molecules for the reactions they catalyze. For eukaryotic, fungal and protozoan glycosyltransferases these are the nucleotide sugars UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-galactose (UDP-Gal), GDP-fucose (GDP-Fuc), CMP-sialicacid (CMP-Sia), UDP-glucuronicacid (UDP-GlcA), GDP-mannose (GDP-Man), and UDP-xylose (UDP-Xyl) (Hirschberg et al. 1998). For the kinases, ATP functions as the donor, while for the sulfotransferases, adenosine 3′-phosphate 5′-phosphate (PAPS) acts as the donor (Hirschberg et al. 1998). The active sites of all these enzymes are oriented towards the lumen of the Golgi cisternae. This necessitates the translocation of their donors from the cytosol into the lumenal Golgi compartments. In this chapter we will focus on the structure, function and localization of the Golgi nucleotide sugar transporters (NSTs), and highlight the diseases and developmental defects associated with defective transporters. We direct the reader to several excellent reviews on Golgi transporters for additional details and references (Hirschberg et al. 1998; Berninsone and Hirschberg 2000; Gerardy-Schahn et al. 2001; Handford et al. 2006; Caffaro and Hirschberg 2006).

Gene expression profiling, chromosome assignment and mutational analysis of the porcine Golgi-resident UDP-N-acetylglucosamine transporter SLC35A3

SLC35A3 encodes a Golgi-resident UDP-N-acetylglucosamine transporter. Here, the porcine SLC35A3 gene was assigned to Sus scrofa chromosome 4 (SSC4) by a combination of radiation hybrid and linkage analysis. Expression profiling using real time RT-PCR showed ubiquitous but variable transcription of SLC35A3 in a selection of tissues. The deduced 325 amino acid sequence revealed a hydrophobic protein with 10 predicted transmembrane helices and the N- and C-terminal tails facing the cytosolic side of the Golgi apparatus. In addition, mutated versions of the UDP-GlcNAc transporter were analyzed in a yeast complementation assay, which allowed us to identify important domains and amino acid residues. Thus, the N-terminal tail was inessential for activity, whereas removal of the first transmembrane domain inhibited yeast complementation. The hydrophilic C-terminus was dispensable while mutant proteins either fully or partially deprived of the last membrane-spanning helix were functionally impaired. The third luminal loop showed modest sequence conservation and appeared structurally flexible as certain deletions were acceptable. In contrast, the fourth luminal loop was more sensitive to changes since the competence of the mutant protein was lowered by mutations. Substitutions of glycines 190, 215 and 254, which are invariant positions in the SLC35A subfamilies affected activity negatively. Interestingly, inhibition of function by a valine to phenylalanine mutation, which has been associated with skeletal malformations, is likely caused by structural incompatibility of the bulky aromatic phenylalanine side chain with the integrity of the transmembrane helix, since substitutions with the smaller aliphatic side chains of leucine and isoleucine were acceptable changes.

Nucleotide-sugar transporters: structure, function and roles in vivo

The glycosylation of glycoconjugates and the biosynthesis of polysaccharides depend on nucleotide-sugars which are the substrates for glycosyltransferases. A large proportion of these enzymes are located within the lumen of the Golgi apparatus as well as the endoplasmic reticulum, while many of the nucleotide-sugars are synthesized in the cytosol. Thus, nucleotide-sugars are translocated from the cytosol to the lumen of the Golgi apparatus and endoplasmic reticulum by multiple spanning domain proteins known as nucleotide-sugar transporters (NSTs). These proteins were first identified biochemically and some of them were cloned by complementation of mutants. Genome and expressed sequence tag sequencing allowed the identification of a number of sequences that may encode for NSTs in different organisms. The functional characterization of some of these genes has shown that some of them can be highly specific in their substrate specificity while others can utilize up to three different nucleotide-sugars containing the same nucleotide. Mutations in genes encoding for NSTs can lead to changes in development in Drosophila melanogaster or Caenorhabditis elegans, as well as alterations in the infectivity of Leishmania donovani. In humans, the mutation of a GDP-fucose transporter is responsible for an impaired immune response as well as retarded growth. These results suggest that, even though there appear to be a fair number of genes encoding for NSTs, they are not functionally redundant and seem to play specific roles in glycosylation.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Nucleotide sugar transporters of the Golgi apparatus: from basic science to diseases

Approximately 80% of secreted and membrane proteins (40% of all proteins) of eukaryotes become covalently linked to sugars in the lumen of the Golgi apparatus, a cellular organelle that is part of the secretory system of all eukaryotes. The sugar donors are mostly nucleoside diphosphate sugars (nucleotide sugars) and must be translocated from the cytosol, their site of synthesis, across the Golgi apparatus membrane and into the lumen by specific transporters. These are hydrophobic, homodimeric proteins that span the membrane multiple times. Mutants of these proteins have developmental phenotypes including diseases in humans and cattle.

Independent and simultaneous translocation of two substrates by a nucleotide sugar transporter

Nucleotide sugar transporters play an essential role in protein and lipid glycosylation, and mutations can result in developmental phenotypes. We have characterized a transporter of UDP-N-acetylglucosamine and UDP-N-acetylgalactosamine encoded by the Caenorhabditis elegans gene C03H5.2. Surprisingly, translocation of these substrates occurs in an independent and simultaneous manner that is neither a competitive nor a symport transport. Incubations of Golgi apparatus vesicles of Saccharomyces cerevisiae expressing C03H5.2 protein with these nucleotide sugars labeled with (3)H and (14)C in their sugars showed that both substrates enter the lumen to the same extent, whether or not they are incubated alone or in the presence of a 10-fold excess of the other nucleotide sugar. Vesicles containing a deletion mutant of the C03H5.2 protein transport UDP-N-acetylglucosamine at rates comparable with that of wild-type transporter, whereas transport of UDP-N-acetylgalactosamine was decreased by 85-90%, resulting in an asymmetrical loss of substrate transport.

Characterization of AtNST-KT1, a novel UDP-galactose transporter from Arabidopsis thaliana

Nucleotide sugar transporters (NST) mediate the transfer of nucleotide sugars from the cytosol into the lumen of the endoplasmatic reticulum and the Golgi apparatus. Because the NSTs show similarities with the plastidic phosphate translocators (pPTs), these proteins were grouped into the TPT/NST superfamily. In this study, a member of the NST-KT family, AtNST-KT1, was functionally characterized by expression of the corresponding cDNA in yeast cells and subsequent transport experiments. The histidine-tagged protein was purified by affinity chromatography and reconstituted into proteoliposomes. The substrate specificity of AtNST-KT1 was determined by measuring the import of radiolabelled nucleotide mono phosphates into liposomes preloaded with various unlabelled nucleotide sugars. This approach has the advantage that only one substrate has to be used in a radioactively labelled form while all the nucleotide sugars can be provided unlabelled. It turned out that AtNST-KT1 represents a monospecific NST transporting UMP in counterexchange with UDP-Gal but did not transport other nucleotide sugars. The AtNST-KT1 gene is ubiquitously expressed in all tissues. AtNST-KT1 is localized to Golgi membranes. Thus, AtNST-KT1 is most probably involved in the synthesis of galactose-containing glyco-conjugates in plants.

Molecular Cloning and Characterization of a Novel 3′-Phosphoadenosine 5′-Phosphosulfate Transporter, PAPST2

Sulfation is an important posttranslational modification associated with a variety of molecules. It requires the involvement of the high energy form of the universal sulfate donor, 3'-phosphoadenosine 5'-phosphosulfate (PAPS). Recently, we identified a PAPS transporter gene in both humans and Drosophila. Although human colonic epithelial tissues express many sulfated glycoconjugates, PAPST1 expression in the colon is trace. In the present study, we identified a novel human PAPS transporter gene that is closely related to human PAPST1. This gene, called PAPST2, is predominantly expressed in human colon tissues. The PAPST2 protein is localized on the Golgi apparatus in a manner similar to the PAPST1 protein. By using yeast expression studies, PAPST2 protein was shown to have PAPS transport activity with an apparent Km value of 2.2 microM, which is comparable with that of PAPST1 (0.8 microM). Overexpression of either the PAPST1 or PAPST2 gene increased PAPS transport activity in human colon cancer HCT116 cells. The RNA interference of the PAPST2 gene in the HCT116 cells significantly reduced the reactivity of G72 antibody directed against the sialyl 6-sulfo N-acetyllactosamine epitope and total sulfate incorporation into cellular proteins. These findings indicate that PAPST2 is a PAPS transporter gene involved in the synthesis of sulfated glycoconjugates in the colon.

Characterization of a mutation and an alternative splicing of UDP-galactose transporter in MDCK-RCAr cell line

The UDP-galactose (UDP-Gal) transporter present in the Golgi apparatus is a member of a transporter family comprising hydrophobic proteins with multiple transmembrane domains. Co-immunoprecipitation experiments showed that the full-length UDP-Gal transporter protein forms oligomeric structures in the MDCK cell. A ricin-resistant mutant of the MDCK cell line (MDCK-RCA(r)) is deficient in galactose linked to macromolecules because of a lower UDP-Gal transport rate into the Golgi apparatus. We cloned this mutated protein and found that it contains a stop codon close to the 5' terminus of its open reading frame. We also detected a shorter splicing variant of the UDP-Gal transporter which contains a 183-nt in-frame deletion in both the wild-type and the mutant mRNA. We showed that the protein, when overexpressed, is localized in the Golgi apparatus and could partially correct the phenotype of the MDCK-RCA(r) and CHO-Lec8 mutant cell lines. The level of mRNA of the UDP-Gal transporter is much lower (25-30 copies per cell) than those of the CMP-sialic acid transporter (100 copies per cell), UDP-N-acetylglucosamine transporter (80 copies per cell), and GDP-fucose transporter (65 copies per cell). The transcript level of the shorter splicing variant of the UDP-Gal transporter is extremely rare in wild-type MDCK cells (a few copies per cell), but it is significantly increased in the mutant, RCA-resistant cells.

A missense mutation in the bovine SLC35A3 gene, encoding a UDP-N-acetylglucosamine transporter, causes complex vertebral malformation

The extensive use of a limited number of elite bulls in cattle breeding can lead to rapid spread of recessively inherited disorders. A recent example is the globally distributed syndrome Complex Vertebral Malformation (CVM), which is characterized by misshapen and fused vertebrae around the cervico-thoracic junction. Here, we show that CVM is caused by a mutation in the Golgi-resident nucleotide-sugar transporter encoded by SLC35A3. Thus, the disease showed complete cosegregation with the mutation in a homozygous state, and proteome patterns indicated abnormal protein glycosylation in tissues of affected animals. In addition, a yeast mutant that is deficient in the transport of UDP-N-acetylglucosamine into its Golgi lumen can be rescued by the wild-type SLC35A3 gene, but not by the mutated gene. These results provide the first demonstration of a genetic disorder associated with a defective SLC35A3 gene, and reveal a new mechanism for malformation of the vertebral column caused by abnormal nucleotide-sugar transport into the Golgi apparatus.

The human solute carrier gene SLC35B4 encodes a bifunctional nucleotide sugar transporter with specificity for UDP-xylose and UDP-N-acetylglucosamine

The transport of nucleotide sugars from the cytoplasm into the Golgi apparatus is mediated by specialized type III proteins, the nucleotide sugar transporters (NSTs). Transport assays carried out in vitro with Golgi vesicles from mammalian cells showed specific uptake for a total of eight nucleotide sugars. When this study was started, NSTs with transport activities for all but two nucleotide sugars (UDP-Xyl and UDP-Glc) had been cloned. Aiming at identifying these elusive NSTs, bioinformatic methods were used to display putative NST sequences in the human genome. Ten open reading frames were identified, cloned, and heterologously expressed in yeast. Transport capabilities for UDP-Glc and UDP-Xyl were determined with Golgi vesicles isolated from transformed cells. Although a potential UDP-Glc transporter could not be identified due to the high endogenous transport background, the measurement of UDP-Xyl transport was possible on a zero background. Vesicles from yeast cells expressing the human gene SLC35B4 showed specific uptake of UDP-Xyl, and subsequent testing of other nucleotide sugars revealed a second activity for UDP-GlcNAc. Expression of the epitope-tagged SLC35B4 in mammalian cells demonstrated strict Golgi localization. Because decarboxylation of UDP-GlcA is known to produce UDP-Xyl directly in the endoplasmic reticulum and Golgi lumen, our data demonstrate that two ways exist to deliver UDP-Xyl to the Golgi apparatus.

Identification and characterization of human Golgi nucleotide sugar transporter SLC35D2, a novel member of the SLC35 nucleotide sugar transporter family

We report the molecular cloning of SLC35D2, a novel member of the SLC35 nucleotide sugar transporter family. The gene SLC35D2 maps to chromosome 9q22.33. SLC35D2 cDNA codes for a hydrophobic protein consisting of 337 amino acid residues with 10 putative transmembrane helices. Northern blot analysis revealed the SLC35D2 mRNA as a single major band corresponding to 2.0 kb in length. SLC35D2 was localized in the Golgi membrane and exhibited around 50% similarity with three nucleotide sugar transporters: human SLC35D1 (UDP-glucuronic acid/UDP-N-acetylgalactosamine transporter), fruitfly fringe connection (frc) transporter, and nematode SQV-7 transporter, the latter two being involved in developmental and organogenetic processes. Heterologous expression of SLC35D2 protein in yeast indicated that UDP-N-acetylglucosamine is a candidate for the substrate(s) of the transporter. The sequence similarity, subcellular localization, and transporting substrate suggest that SLC35D2 is a good candidate for the ortholog of frc transporter, which is involved in the Notch signaling system by providing the fringe N-acetylglucosaminyltransferase with the substrate. We also describe the identification and categorization of the human SLC35 gene family.

Reconstitution of GDP-mannose transport activity with purified Leishmania LPG2 protein in liposomes

Activated nucleotide sugars required for the synthesis of glycoconjugates within the secretory pathway of eukaryotes are provided by the action of nucleotide sugar transporters (NSTs). Typically, NSTs are studied in microsomal preparations from wild-type or mutant lines; however, in this setting it can be difficult to assess NST properties because of the presence of glycosyltransferases and other interfering activities. Here we have engineered Leishmania donovani to express high levels of an active LPG2 Golgi GDP-Man transporter bearing a C-terminal polyhistidine tag. The functional LPG2-HIS was solubilized, purified by metal affinity chromatography, and reconstituted into phosphatidylcholine-containing liposomes using polystyrene SM-2 beads. The proteoliposomes exhibited robust GDP-Man transport activity with an apparent K(m) of 6.6 mum. Transport activity was enhanced by preloading of GMP and showed specificity for multiple substrates (GDP-Ara and GDP-Fuc). In contrast to the activity in crude microsomes, transport was not dependent on the presence of divalent cations. Thus, reconstitution of transport activity using purified LPG2 protein in liposomes provides firm experimental evidence that a single polypeptide is solely required for NST activity and is able to mediate the uptake of multiple substrates. These studies are relevant to the study of NST structure and function in both protozoan parasites as well as their higher eukaryotic hosts.

Localization of GDP-mannose transporter in the Golgi requires retrieval to the endoplasmic reticulum depending on its cytoplasmic tail and coatomer

The Saccharomyces cerevisiae GDP-mannose transporter (GMT) encoded by the essential gene VRG4/VIG4 is a member of the nucleotide-sugar transporter family in the Golgi apparatus. We examined GMT in the secretory mutant cells to investigate the mechanism of its localization in the Golgi. At the nonpermissive temperature, most GMT was found in the endoplasmic reticulum of sec23ts cells, which have defective COPII, and in the vacuole of sec21ts cells, which have defective COPI. The C-terminal hydrophilic peptide of GMT that is exposed to the cytosol binds to Ret2p, a subunit of the COPI coat. Mutant peptide derivatives that have lost a cluster of lysine in the vicinity of the transmembrane domain had reduced binding activity to Ret2p and the GMT with this sequence was delivered to the vacuole. Our results indicate that GMT escapes from delivery to the vacuole by recycling to the endoplasmic reticulum and retrieval requires the lysine-rich C-terminal tail that can bind to the COPI coat.

Improved production of heterologous proteins by a glucose repression-defective mutant of Kluyveromyces lactis

The secreted production of heterologous proteins in Kluyveromyces lactis was studied. A glucoamylase (GAA) from the yeast Arxula adeninivorans was used as a reporter protein for the study of the secretion efficiencies of several wild-type and mutant strains of K. lactis. The expression of the reporter protein was placed under the control of the strong promoter of the glyceraldehyde-3-phosphate dehydrogenase of Saccharomyces cerevisiae. Among the laboratory strains tested, strain JA6 was the best producer of GAA. Since this strain is known to be highly sensitive to glucose repression and since this is an undesired trait for biomass-oriented applications, we examined heterologous protein production by using glucose repression-defective mutants isolated from this strain. One of them, a mutant carrying a dgr151-1 mutation, showed a significantly improved capability of producing heterologous proteins such as GAA, human serum albumin, and human interleukin-1beta compared to the parent strain. dgr151-1 is an allele of RAG5, the gene encoding the only hexokinase present in K. lactis (a homologue of S. cerevisiae HXK2). The mutation in this strain was mapped to nucleotide position +527, resulting in a change from glycine to aspartic acid within the highly conserved kinase domain. Cells carrying the dgr151-1 allele also showed a reduction in N- and O-glycosylation. Therefore, the dgr151 strain may be a promising host for the production of heterologous proteins, especially when the hyperglycosylation of recombinant proteins must be avoided.

Alterations of O-glycosylation, cell wall, and mitochondrial metabolism in Kluyveromyces lactis cells defective in KlPmr1p, the Golgi Ca2+-ATPase

In yeast the P-type Ca(2+)-ATPase of the Golgi apparatus, Pmr1p, is the most important player in calcium homeostasis. In Kluyveromyces lactis KlPMR1 inactivation leads to pleiotropic phenotypes, including reduced N-glycosylation and altered cell wall morphogenesis. To study the physiology of K. lactis when KlPMR1 was inactivated microarrays containing all Saccharomyces cerevisiae coding sequences were utilized. Alterations in O-glycosylation, consistent with the repression of KlPMT2, were found and a terminal N-acetylglucosamine in the O-glycans was identified. Klpmr1Delta cells showed increased expression of PIRs, proteins involved in cell wall maintenance, suggesting that responses to cell wall weakening take place in K. lactis. We found over-expression of KlPDA1 and KlACS2 genes involved in the Acetyl-CoA synthesis and down-regulation of KlIDP1, KlACO1, and KlSDH2 genes involved in respiratory metabolism. Increases in oxygen consumption and succinate dehydrogenase activity were also observed in mutant cells. The described approach highlighted the unexpected involvement of KlPMR1 in energy-yielding processes.

Loss of srf-3-encoded nucleotide sugar transporter activity in Caenorhabditis elegans alters surface antigenicity and prevents bacterial adherence

During the establishment of a bacterial infection, the surface molecules of the host organism are of particular importance, since they mediate the first contact with the pathogen. In Caenorhabditis elegans, mutations in the srf-3 locus confer resistance to infection by Microbacterium nematophilum, and they also prevent biofilm formation by Yersinia pseudotuberculosis, a close relative of the bubonic plague agent Yersinia pestis. We cloned srf-3 and found that it encodes a multitransmembrane hydrophobic protein resembling nucleotide sugar transporters of the Golgi apparatus membrane. srf-3 is exclusively expressed in secretory cells, consistent with its proposed function in cuticle/surface modification. We demonstrate that SRF-3 can function as a nucleotide sugar transporter in heterologous in vitro and in vivo systems. UDP-galactose and UDP-N-acetylglucosamine are substrates for SRF-3. We propose that the inability of Yersinia biofilms and M. nematophilum to adhere to the nematode cuticle is due to an altered glycoconjugate surface composition of the srf-3 mutant.

KlPMR1 inactivation and calcium addition enhance secretion of non-hyperglycosylated heterologous proteins in Kluyveromyces lactis

The Kluyveromyces lactis KlPMR1 gene is the functional homologue of Saccharomyces cerevisiae PMR1 which encodes a Ca(2+)-ATPase localized in the Golgi apparatus. We studied the effects of KlPMR1 inactivation on the glycosylation and secretion of native and heterologous proteins in K. lactis. We used acid phosphatase, recombinant human serum albumin and alpha-glucoamylase from Arxula adeninivorans as reporter proteins. The Klpmr1Delta strain showed enhanced secretion of the heterologous proteins analyzed; the improved rHSA production did not result from enhanced transcription but rather involved increased translation and/or secretion efficiency. The growth rate of mutant cells resulted slower as compared to that of wild-type strain. The addition of 10mM calcium to the culture medium, however, not only completely relieved the growth defect of the mutant cells but also improved the rate of heterologous proteins production. Moreover, the addition of this ion in the culture medium of K. lactis did not suppress the glycosylation defects; this is an important difference with respect to S. cerevisiae where the glycosylation is partially restored by Ca(2+) addition. The Klpmr1Delta strain as a host offers thus an additional advantage for those cases requiring that the produced recombinant protein would not result hyperglycosylated.

Molecular physiology and pathology of the nucleotide sugar transporter family (SLC35)

The solute carrier family SLC35 consists of at least 17 molecular species in humans. The family members so far characterized encode nucleotide sugar transporters localizing at the Golgi apparatus and/or the endoplasmic reticulum (ER). These transporters transport nucleotide sugars pooled in the cytosol into the lumen of these organelles, where most glycoconjugate synthesis occurs. Pathological analyses and developmental studies of small, multicellular organisms deficient in nucleotide sugar transporters have shown these transporters to be involved in tumour metastasis, cellular immunity, organogenesis and morphogenesis. Leukocyte adhesion deficiency type II (LAD II) or the congenital disorder of glycosylation type IIc (CDG IIc) are the sole human congenital disorders known to date that are caused by a defect of GDP-fucose transport. Along with LAD II, the possible involvement of nucleotide sugar transporters in disorders of connective tissues and muscles is also discussed.

Cell wall-associated enzymes in fungi

This review compiles and discusses previous reports on the identity of wall-associated enzymes (WAEs) in fungi and addresses critically the widely different terminologies used in the literature to specify the type of bonding of WAEs to other entities of the cell wall compartment, the extracellular matrix (ECM). A facile and rapid fractionation protocol for catalytically active WAEs is presented, which uses crude cell walls as the experimental material, a variety of test enzymes (including representatives of polysaccharide synthases and hydrolases, phosphatases, gamma-glutamyltransferases, pyridine-nucleotide dehydrogenases and phenol-oxidising enzymes) and a combination of simple hydrophilic and hydrophobic extractants. The protocol provides four fully operationally defined classes of WAEs, with constituent members of each class displaying the same basic type of physicochemical interaction with binding partners in situ. The routine application of the protocol to different species and cell types could yield easily accessible data useful for building-up a general objective information retrieval system of WAEs, suitable as an heuristic basis both for the unravelling of the role and for the biotechnological potentialities of WAEs. A detailed account is given of the function played in the ECM by WAEs in the metabolism of chitin (chitin synthase, chitinase and beta-N-acetylhexosaminidase) and of phenols (tyrosinase).

The nucleotide-sugar transporter family: a phylogenetic approach

Nucleotide sugar transporters (NST) establish the functional link of membrane transport between the nucleotide sugars synthesized in the cytoplasm and nucleus, and the glycosylation processes that take place in the endoplasmic reticulum (ER) and Golgi apparatus. The aim of the present work was to perform a phylogenetic analysis of 87 bank annotated protein sequences comprising all the NST so far characterized and their homologues retrieved by BLAST searches, as well as the closely related triose-phosphate translocator (TPT) plant family. NST were classified in three comprehensive families by linking them to the available experimental data. This enabled us to point out both the possible ER subcellular targeting of these transporters mediated by the dy-lysine motif and the substrate recognition mechanisms specific to each family as well as an important acceptor site motif, establishing the role of evolution in the functional properties of each NST family.

Human and Drosophila UDP-galactose transporters transport UDP-N-acetylgalactosamine in addition to UDP-galactose

A putative Drosophila nucleotide sugar transporter was characterized and shown to be the Drosophila homologue of the human UDP-Gal transporter (hUGT). When the Drosophila melanogaster UDP-Gal transporter (DmUGT) was expressed in mammalian cells, the transporter protein was localized in the Golgi membranes and complemented the UDP-Gal transport deficiency of Lec8 cells but not the CMP-Sia transport deficiency of Lec2 cells. DmUGT and hUGT were expressed in Saccharomyces cerevisiae cells in functionally active forms. Using microsomal vesicles isolated from Saccharomyces cerevisiae expressing these transporters, we unexpectedly found that both hUGT and DmUGT could transport UDP-GalNAc as well as UDP-Gal. When amino-acid residues that are conserved among human, murine, fission yeast and Drosophila UGTs, but are distinct from corresponding ones conserved among CMP-Sia transporters (CSTs), were substituted by those found in CST, the mutant transporters were still active in transporting UDP-Gal. One of these mutants in which Asn47 was substituted by Ala showed aberrant intracellular distribution with concomitant destabilization of the protein product. However, this mutation was suppressed by an Ile51 to Thr second-site mutation. Both residues were localized within the first transmembrane helix, suggesting that the structure of the helix contributes to the stabilization and substrate recognition of the UGT molecule.

Identification and characterization of GONST1, a golgi-localized GDP-mannose transporter in Arabidopsis

Transport of nucleotide sugars across the Golgi apparatus membrane is required for the luminal synthesis of a variety of plant cell surface components. We identified an Arabidopsis gene encoding a nucleotide sugar transporter (designated GONST1) that we have shown by transient gene expression to be localized to the Golgi. GONST1 complemented a GDP-mannose transport-defective yeast mutant (vrg4-2), and Golgi-rich vesicles from the complemented strain displayed increased GDP-mannose transport activity. GONST1 promoter::beta-glucuronidase studies suggested that this gene is expressed ubiquitously. The identification of a Golgi-localized nucleotide sugar transporter from plants will allow the study of the importance of this class of proteins in the synthesis of plant cell surface components such as cell wall polysaccharides.

Identification and characterization of GONST1, a golgi-localized GDP-mannose transporter in Arabidopsis

Transport of nucleotide sugars across the Golgi apparatus membrane is required for the luminal synthesis of a variety of plant cell surface components. We identified an Arabidopsis gene encoding a nucleotide sugar transporter (designated GONST1) that we have shown by transient gene expression to be localized to the Golgi. GONST1 complemented a GDP-mannose transport-defective yeast mutant (vrg4-2), and Golgi-rich vesicles from the complemented strain displayed increased GDP-mannose transport activity. GONST1 promoter::beta-glucuronidase studies suggested that this gene is expressed ubiquitously. The identification of a Golgi-localized nucleotide sugar transporter from plants will allow the study of the importance of this class of proteins in the synthesis of plant cell surface components such as cell wall polysaccharides.

Nucleotide sugar transporters: biological and functional aspects

The Golgi apparatus serves as the major site of glycosylation reactions. Nucleotide sugars which are substrates of the Golgi localized glycosyltransferases are synthesized in the cytoplasm (cell nucleus in case of CMP-sialic acid) and must be transported into the compartment lumen. This transport function is carried out by nucleotide sugar transporters. The first genes were cloned in the year 1996 and revealed a family of structurally conserved multi-transmembrane-spanning proteins. Due to the high structural and functional conservation, the identification of many putative nucleotide sugar transporter sequences has become possible in the existing gene data bases and accelerates the increase in knowledge on structure-function-relationships. Recent developments in the nucleotide sugar transporter field are discussed in this article.

The drug/metabolite transporter superfamily

Previous work defined several families of secondary active transporters, including the prokaryotic small multidrug resistance (SMR) and rhamnose transporter (RhaT) families as well as the eukaryotic organellar triose phosphate transporter (TPT) and nucleotide-sugar transporter (NST) families. We show that these families as well as several other previously unrecognized families of established or putative secondary active transporters comprise a large ubiquitous superfamily found in bacteria, archaea and eukaryotes. We have designated it the drug/metabolite transporter (DMT) superfamily (transporter classification number 2.A.7) and have shown that it consists of 14 phylogenetic families, five of which include no functionally well-characterized members. The largest family in the DMT superfamily, the drug/metabolite exporter (DME) family, consists of over 100 sequenced members, several of which have been implicated in metabolite export. Each DMT family consists of proteins with a distinctive topology: four, five, nine or 10 putative transmembrane alpha helical spanners (TMSs) per polypeptide chain. The five TMS proteins include an N-terminal TMS lacking the four TMS proteins. The full-length proteins of 10 putative TMSs apparently arose by intragenic duplication of an element encoding a primordial five-TMS polypeptide. Sequenced members of the 14 families are tabulated and phylogenetic trees for all the families are presented. Sequence and topological analyses allow structural and functional predictions.

Substrate recognition by UDP-galactose and CMP-sialic acid transporters. Different sets of transmembrane helices are utilized for the specific recognition of UDP-galactose and CMP-sialic acid

Human UDP-galactose transporter (hUGT1) and CMP-sialic acid transporter (hCST) are related Golgi membrane proteins with 10 transmembrane helices. We have constructed chimeras between these proteins in order to identify submolecular regions responsible for the determination of substrate specificity. To assess the UGT and CST activities, chimeric cDNAs were transiently expressed in either UGT-deficient mutant Lec8 cells or CST-deficient mutant Lec2 cells, and the binding of plant lectins, GS-II or PNA, respectively, to these cells was examined. During the course of analysis of various chimeric transporters, we found that chimeras whose submolecular regions contained helices 1, 8, 9, and 10, and helices 2, 3, and 7 derived from hUGT1 and hCST sequences, respectively, exhibited both UGT and CST activities. The dual substrate specificity for UDP-galactose and CMP-sialic acid of one such representative chimera was directly confirmed by in vitro measurement of the nucleotide sugar transport activity using a heterologous expression system in the yeast Saccharomyces cerevisiae. These findings indicated that the regions which are critical for determining the substrate specificity of UGT and CST resided in different submolecular sites in the two transporters, and that these different determinants could be present within one protein without interfering with each other's function.

Functional characterization of Gms1p/UDP-galactose transporter in Schizosaccharomyces pombe

Galactosylation of glycoproteins in the fission yeast Schizosaccharomyces pombe requires the transport of UDP-galactose as substrate for the galactosyltransferase into the lumen of the Golgi apparatus, which is achieved by the UDP-galactose transporter. We isolated a mutant (gms1) that is deficient in galactosylation of cell surface glycoproteins in Sz.pombe, and found that the gms1(+) gene encodes a UDP-galactose transporter. In the prediction of secondary structure of the Gms1 protein, an eight-membrane-spanning structure was obtained. Fluorescent microscopy revealed the functional Gms1-GFP fusion protein to be stably localized at the Golgi membrane. Sequencing analysis of the coding region of Gms1p derived from galactosylation-defective mutants identified a single amino acid mutation (A102T or A258E) located within the putative transmembrane region, helix 2 or helix 7, respectively. The mutagenized Gms1(A102T or A258E)p exhibited loss of UDP-galactose transport activity but no change in the localization to the Golgi membrane. The C-terminal truncated Gms1p mutants demonstrated that the C-terminal hydrophilic region was dispensable for targeting and function as UDP-galactose transporter at the Golgi membrane. We suggest that the putative eighth (the most C-terminus-proximal) transmembrane helix of Gms1p is critical to targeting from ER to the Golgi membrane.

The gene defective in leukocyte adhesion deficiency II encodes a putative GDP-fucose transporter

Leukocyte adhesion deficiency II (LAD II) is characterized by the lack of fucosylated glycoconjugates, including selectin ligands, causing immunodeficiency and severe mental and growth retardation. No deficiency in fucosyltransferase activities or in the activities of enzymes involved in GDP-fucose biosynthesis has been found. Instead, the transport of GDP-fucose into isolated Golgi vesicles of LAD II cells appeared to be reduced. To identify the gene mutated in LAD II, we cloned 12 cDNAs from Caenorhabditis elegans, encoding multi-spanning transmembrane proteins with homology to known nucleotide sugar transporters, and transfected them into fibroblasts from an LAD II patient. One of these clones re-established expression of fucosylated glycoconjugates with high efficiency and allowed us to identify a human homolog with 55% identity, which also directed re-expression of fucosylated glycoconjugates. Both proteins were localized to the Golgi. The corresponding endogenous protein in LAD II cells had an R147C amino acid change in the conserved fourth transmembrane region. Overexpression of this mutant protein in cells from a patient with LAD II did not rescue fucosylation, demonstrating that the point mutation affected the activity of the protein. Thus, we have identified the first putative GDP-fucose transporter, which has been highly conserved throughout evolution. A point mutation in its gene is responsible for the disease in this patient with LAD II.

The UDPase activity of the Kluyveromyces lactis Golgi GDPase has a role in uridine nucleotide sugar transport into Golgi vesicles

In Saccharomyces cerevisiae a Golgi lumenal GDPase (ScGda1p) generates GMP, the antiporter required for entry of GDP-mannose, from the cytosol, into the Golgi lumen. Scgda1 deletion strains have severe defects in N- and O-mannosylation of proteins and glycosphingolipids. ScGda1p has also significant UDPase activity even though S. cerevisiae does not utilize uridine nucleotide sugars in its Golgi lumen. Kluyveromyces lactis, a species closely related to S. cerevisiae, transports UDP-N-acetylglucosamine into its Golgi lumen, where it is the sugar donor for terminal N-acetylglucosamine of the mannan chains. We have identified and cloned a K. lactis orthologue of ScGda1p. KlGda1p is 65% identical to ScGda1p and shares four apyrase conserved regions with other nucleoside diphosphatases. KlGda1p has UDPase activity as ScGda1p. Transport of both GDP-mannose, and UDP-GlcNAc was decreased into Golgi vesicles from Klgda1 null mutants, demonstrating that KlGda1p generates both GMP and UMP required as antiporters for guanosine and uridine nucleotide sugar transport into the Golgi lumen. Membranes from Klgda1 null mutants showed inhibition of glycosyltransferases utilizing uridine- and guanosine-nucleotide sugars, presumably due to accumulation of nucleoside diphosphates because the inhibition could be relieved by addition of apyrase to the incubations. KlGDA1 and ScGDA1 restore the wild-type phenotype of the other yeast gda1 deletion mutant. Surprisingly, KlGDA1 has only a role in O-glycosylation in K. lactis but also complements N-glycosylation defects in S. cerevisiae. Deletion mutants of both genes have altered cell wall stability and composition, demonstrating a broader role for the above enzymes.